Flexible circuits commonly called Flex circuits consist of metallic circuit traces that are built onto Mylar through a photo imaging & etching process onto which electronic components are frequently mounted or attached. Usually, to minimize cost and enhance flexibility Flex circuits are built with one or two trace layers that contain electrically conductive metallic traces. Furthermore the metallic traces are usually encapsulated in Mylar that acts as a solder mask in order to minimize shorting from external contamination. Furthermore exposed metallic contacts, vias or through holes are usually plated with solder such that exposed metallic surfaces resist corrosion.
NOTE: In this document the terms “trace layer” and “trace layers” are used to identify a single flexible circuit that contains a number of independent signal carrying metallic layers that are encapsulated together as a single unit: These terms are used to describe conventional flexible circuits.
Flex circuits can also be built with more than two trace layers, in ways that are similar to how multiple layer circuit boards are constructed. Building Flex circuits that contain more than two trace layers is less flexible and more expensive than Flex circuits that contain one or two trace layers as each added trace layer increases the rigidity of the Flex circuit. A defect in any trace layer of a multiple trace layers Flex circuit may cause the entire Flex circuit to be scrapped.
Flexible circuits are also built into sheets. Sometimes these sheets are designed to be folded after electronic components are attached to the flex circuit. This folding of Flex circuits impose significant limitations on designers, however, these limitations include: 1. A single defect in a sheet can cause the entire sheet of Flex circuit to be scraped: 2. Trace lengths increase as the size of the sheet increases: 3. The more layers contained within folded Flex circuits increases stiffness of the Flex circuit, making it more difficult to fold, while increasing the likelihood that traces contained within will crack, or that the Flex circuit will contain defect defects from the manufacture of the raw flex circuit itself: 4. Folded Flex circuits will trap heat as Mylar is a thermal insulator, heat is the enemy of electronic components.
Since Flex circuits are excellent thermal insulators any 3 dimensional electronic assembly built using conventional techniques will not keep electronic components cool as heat will not be transferred away from the component readily.
If however distinct separate Flexible circuits are built using a series of small sheets to which electronic components are attached, soldered, or bonded; and then then assembled into a multiple layer Flex circuit with embedded components and designed to transfer heat many of the limitations of using Flex circuits can be overcome. Thus the invention described within is designed to overcome the limitations of conventional Flex circuits in unique non-obvious ways.
Other consideration are 3 dimensional electronic circuits that are built by stacking a plurality of electronic devices, sometimes various layers of electronic devices are wire bonded together, or may be built by soldering electronic components or die directly to each other. The electrical interconnections in these types of three dimensional electronic circuits are stressed and can crack when exposed to changing temperature or thermal cycling. Furthermore these structures are not designed to efficiently transfer heat away from the electronic components contained within the assembly.
Thus invention described within this document is an electronic assembly that uses multiple flexible circuits that are assembled into unique configurations with unique attributes that resolve shortcomings of conventional Flex circuit and conventional 3 dimensional electronic assemblies.
Such a device includes: a plurality of flexible circuits with electronic components attached, where each flexible circuit is separated by substances that are designed to transfer heat away from electronic components that are located between different flexible circuits forming a “multi-layer” 3 dimensional “layered Flex circuit assembly” from a plurality of “Flex circuit sub-assemblies”. Thus the invention describe within is substantially different from conventional Flex circuits.
Substances that may be used to transfer heat include: heat sinks made of metal or carbon: heat conductive pads that fill voids and make heat sinks more efficient: phase change materials: conduction of heat through a gas: forced air blowing over a hot spot: flowing of a chilled fluid: conduction of heat through a liquid: and heat pipes where a fluid evaporates into a gas at a hot location carrying heat away to a cooler location.
Furthermore approaches that embed electronic components between layers of flex circuits without a means for moving heat from said embedded components in an efficient way may fail because of high temperature.
Yet the use of layers of flex circuits where various layers are separated by a distance with heat transfer mechanisms built into a 3 dimensional assembly that can efficiently move heat away from embedded components such that their useful life can be maximized is new. Furthermore since each Flex circuit sub-assembly is attached at certain points and the different layers are free to float relative to each other, stresses components are not subjected to mechanical stresses from induced by other layers or from thermal expansion and contraction.
A non-obvious benefit of such a layered Flex circuit assembly where components are soldered onto flex circuits and assembled within a liquid filled enclosure or heat pipe is that the electronic assembly itself can be soldered in an oven to a circuit board without melting the solder that attaches the electronic components to the Flex circuit sub-assemblies; as the liquid in such a case prevents the temperature inside the enclosure from rising quickly.